Screwed

TheRaven asked about the propeller (screw) on the Virginia model and caught me out — it’s a placeholder borrowed from another model. I’m still digging for decent drawings of Virginia’s screw, which would have been her original, fitted when she was in service as U.S.S. Merrimac.

In the meantime, here’s the screw from C.S.S. Alabama, the high-seas raider built by Laird at Birkenhead in 1862. Although fitted out as a steamship, Alabama was intended to spend most of her time at sea under sail to conserve fuel, and so (like many ships of that period) was fitted with a “lifting” screw that could be disconnected from the shaft and hoisted almost entirely out of the water to reduce drag. Alabama‘s screw was 14 feet 3 inches (4.34m) in diameter. This model is based on plans in Andrew Bowcock’s authoritative C.S.S. Alabama: Anatomy of a Confederate Raider, which in turn are based on shipyard-built presentation model of the ship. Ship’s trials data for Alabama are missing, but based on the models and builders’ trials of contemporary vessels, Bowcock suggests that Alabama‘s screw had a pitch angle of 35°, a “slip” — a measure of the difference between the distance that a propeller of a given pitch should have moved forward (as the result of a full rotation) and the distance that it actually moved forward — of 15-20%, and operated at full speed at 60-65 rpm. Alabama‘s screw would have been more technically refined that Merrimac/Virginia‘s, but closer than the one shown in the renders below.

There was a lot more “wow” stuff to the Alabama as well….her stack could be lowered below deck to make her look different from a distance…and she had a desalination system to provide “fresh” water. Quite a ship.
Tim